[Karbonn A21] bgb7:
This is the first ever guide written by me, and I am not an expert in android and recovery, so please pardon if any mistakes are found. Lets correct if any. Also at this time Karbonn A21's system.img file is not available hence we had to perform some hard steps to get the job done.

CAUTION : WARRANTY VOID : I'm not responsible if any hard/soft damage occurs to your handset after/during using this guide. I successfully unbricked my A21 and want all of you to understand the recovery procedure properly before doing anything. Rooting your handset and applying another recovery/ROM certainly voids manufacturer's warranty. Do anything mentioned here ONLY if you understand it.

The only reason I posted this is to save any karbonn a21 users time if his device gets bricked. I had to keep my business aside for a whole day to recover my Karbonn A21 in working state.

You will have to keep handy :

1) Drivers for Karbonn A21 Click here

1) CWM by Eklovya Click here

2) AIO Flasher by Fonehacker Click here

3) Custom ROM flashable reloaded (*.zip) and A9+ system image (system.img) (because A21 's system image was not found at this time.) here and here

Also use Root_with_Restore_by_Bin4ry_v18 to root your karbonn a21 whenever necessary.

First of all you must understand that three partitions are necessary in Android : 1) Boot, 2) System, 3) Recovery

Other than these, there are Data, Cache, etc.

Unbricking an Android handset involves

1) Repairing Boot partition in fastboot mode using AIO Flasher to flash a proper boot.img file to the handset.

2) Installing proper Recovery partition in fastboot or normal mode using AIO Flasher, CWM Recovery by eklovya is a suitable solution.

3) Installing the System partition from a proper system.img file using AIO Flasher, or from a .zip file (Custom ROM) or a backed up ROM through CWM recovery.

Fastboot mode :

Fastboot mode is a Android device's state where it is possible to flash different partitions through a simple USB cable and PC. It can be activated on Karbonn A21 by keep pressed [Volume Up] key during power on (read : before power on) until your screen glows in green colour. Instantly release [Volume Up] key or your A21 may boot into recovery partition. On your PC proper drivers must be installed and fastboot.exe from Google is required to flash the partitions to the handset. AIO Flasher is a good GUI based solution for that.

Recovery :

During the above process, (for Karbonn A21) if you keep the [Volume Up] key pressed until the screen glows red, and then release the key, you enter into recovery mode. Updates to operating system's files can be installed from this mode. Stock recovery means manufacturer provided Recovery partition 'as it is' and Custom recovery is recovery partition OS developed by other developers, CWM recovery being the most popular one.

STEPS FOR UNBRICKING :

1) Repair Boot partition

Firstly enter into fastboot mode, and then use AIO Flasher to flash boot.img of Karbonn A9+ / Cherry Mobile flare to boot partition of your A21

2) Repair Recovery partition

Using AIO flasher, flash CWM Recovery's img file to recovery partition, in A21's fastboot mode. (For more info read this and this thread)

3) Install System.img to system partition

Currently A21's system.img is not available, else we could easily flash it to A21's system partition and get our handset running. Hence I flashed A9+ system.img downloaded from above (note that the downloaded file has extension .7z, to be renamed to .img), by AIO Flasher to system partition in fastboot mode. The flashed system rom was made for Karbonn A9+ hence it didn't work on my handset properly. I got white display and the touch screen and buttons didn't work. I used AIO flasher's screenshot facility to get into the handset and found that actually the system was up and running properly, except the display and touch interface. Then I decided to flash custom ROM and tried using santo_rock 's custom ROM DarkBoid. But it didn't work. Then I flashed ashoknraj 's custom ROM Flashable reloaded. It was success.

4) Install ROM for A21

To install the custom ROM via Eklovya's CWM recovery, the .zip file needed to be inside internal memory in the form of update.zip file stored in root folder, but the internal memory was not accessible through my PC as its usb drive was shown in windows explorer but was in [no disk] state, and I could not select [Turn on USB Storage] on my A21.

Here comes ADB 's power. I used ADB push to push the custom ROM into the internal memory.

Use the following command to push the update.zip (rename the custom ROM to update.zip) :

adb push update.zip /mnt/sdcard/sd_card/update.zip

Note the spaces and special characters properly.

The file is greater than 100 MB and hence some time will be required to transfer that file into memory.

If it shows error Permission denied, then remount using the command

ADB REMOUNT

and then try again.

Now we have the custom ROM ready in A21's internal memory, lets flash it. Boot into CWM recovery mode and select apply update.zip, let it install, and then wipe data/cache and reboot the device normally. (For more info read this thread)

If everything goes well, your A21 will now be successfully unbricked.

Of course you need patience and concentration to do all this.

DISCLAIMER : The above process and softwares are not my creation and are of respective developers. I just aggregated all these to one place.